Improve the documentation for `--keep-within`

Move the explanation below the general explanation of the `--keep-*` option
behavior rephrase the last sentence to make it clear that it works like the
other options that were explained in the previous paragraph.

Resolves #7687

(cherry picked from commit a6f452a05023621a84a4bdca31b31a6ea0f2b73a)

docs/usage/prune.rst.inc

@@ -126,12 +126,6 @@ If you have multiple sequences of archives with different data sets (e.g.
 from different machines) in one shared repository, use one prune call per
 data set that matches only the respective archives using the -P option.
 
-The ``--keep-within`` option takes an argument of the form "<int><char>",
-where char is "H", "d", "w", "m", "y". For example, ``--keep-within 2d`` means
-to keep all archives that were created within the past 48 hours.
-"1m" is taken to mean "31d". The archives kept with this option do not
-count towards the totals specified by any other options.
-
 A good procedure is to thin out more and more the older your backups get.
 As an example, ``--keep-daily 7`` means to keep the latest backup on each day,
 up to 7 most recent days with backups (days without backups do not count).
@@ -145,6 +139,13 @@ hourly, daily, weekly, monthly, or yearly rules was not otherwise able to meet
 its retention target. This enables the first chronological archive to continue
 aging until it is replaced by a newer archive that meets the retention criteria.
 
+The ``--keep-within`` option takes an argument of the form "<int><char>",
+where char is "H", "d", "w", "m", "y". For example, ``--keep-within 2d`` means
+to keep all archives that were created within the past 48 hours.
+"1m" is taken to mean "31d". This option is applied before the secondly option
+and like the other options any archives kept by this option do not count towards
+the later rules.
+
 The ``--keep-last N`` option is doing the same as ``--keep-secondly N`` (and it will
 keep the last N archives under the assumption that you do not create more than one
 backup archive in the same second).
@@ -152,4 +153,4 @@ backup archive in the same second).
 When using ``--stats``, you will get some statistics about how much data was
 deleted - the "Deleted data" deduplicated size there is most interesting as
 that is how much your repository will shrink.
-Please note that the "All archives" stats refer to the state after pruning.
+Please note that the "All archives" stats refer to the state after pruning.